Fouling represents a critically important barrier against wider adoption of polymer membranes for many applications, including water purification. See, for example, R. L. Riley, in Membrane Separation Systems—A Research & Development Needs Assessment, R. W. Baker, E. L. Cussler, W. Eykamp, W. J. Koros, R. L. Riley, and H. Strathmann, Eds., Department of Energy: Publication number DOE/ER/30133-H1, Springfield, Va., 1990, pages 5(1)-5(53); L. J. Zeman and A. L. Zydney in Microfiltration and Ultrafiltration Principles and Applications, Marcel Dekker Inc.: N.Y., 1996, pages 397-446; and G. Belfort, R. H. Davis, and A. L. Zydney, J. Membr. Sci., 1994, volume 96, page 1. Surface and inner-membrane fouling by solutes such as proteins, emulsified oil droplets, and various types of particles leads to considerable loss in flux and selectively over time. See, K. Scott in Handbook of Industrial Membranes, 2nd ed., Elsevier: Oxford, 2003. While a number of commodity polymers have been fabricated into useful and commercially viable membranes, new synthetic polymers designed to improve existing membranes are needed to prevent fouling and/or improve separation efficiency, and advance the performance of existing commercial membranes beyond their current state.
The known anti-fouling and surface active properties of polyethylene oxide (PEO; also known as polyethylene glycol (PEG)), are appealing for integration into polymer membranes. See, I. Szleifer, Current Opinion Solid State & Mater Sci. 1997, volume 2, page 337; J. H. Lee, H. B. Lee, J. D. Andrade, Prog. Polym. Sci. 1995, volume 20, page 1043. However, the water solubility and crystallinity of PEO make it unsuitable for fabrication into robust membranes for aqueous applications. Thus, covalent attachment of PEO to hydrophobic polymers provides a means by which PEO-based polymers can be integrated into membranes with anti-fouling character, yet maintain the integrity of the membrane in an aqueous environment. PEO has been attached covalently to a variety of membrane materials, including cellulose (see, T. Akizawa, K. Kino, S. Koshikawa, Y. Ikada, A. Kishida, M. Yamashita, K. Imamura, Trans. Am. Soc. Artif. Intern. Organs 1989, volume 35, page 333), poly(acrylonitrile) (see, M. Ulbricht, H. Matuschewski, A. Oechel, H. G. Hicke, J. Membr. Sci. 1996, volume 115, page 31), poly(acrylonitrile-co-vinyl chloride) (see, M. S. Shoichet, S. R. Winn, S. Athavale, J. M. Harris, F. T. Gentile, Biotechnol. Bioeng. 1994, volume 43, page 563), polysulfone (see, V. Thom, K. Jankova, M. Ulbricht, J. Kops, G. Jonsson, Macromol. Chem. Phys. 1998, volume 199, page 2723; M. A. Harmer, Langmuir 1991, volume 7, page 2010), and polypropylene and cellulose acetate (see, M. Huiman, R. H. Davis, C. N. Bowman, Macromolecules 2000, volume 33, page 331). Such “PEGylated” membranes typically show considerably different properties from the non-PEGylated versions, especially with respect to reduced fouling, and permeability/selectivity characteristics. See, V. Thom, K. Jankova, M. Ulbricht, J. Kops, G. Jonsson, Macromol. Chem. Phys. 1998, volume 199, page 2723; J. F. Hester, A. M. Mayes, J. Membr. Sci. 2002, volume 202, page 119; J. F. Hester, P. Benerjee, Y. Y. Won, A. Akthakul, M. H. Acar, A. M. Mayes, Macromolecules 2002, volume 35, page 7652; H. Iwata, M. I. Ivanchenko, Y. Miyaki, J. Appl. Polym. Sci. 1994, volume 54, page 125. For polysulfone ultrafiltration (UF) membranes, ultraviolet light-induced reactions of PEG derivatives have been used for membrane surface modification. See, V. Thom, K. Jankova, M. Ulbricht, J. Kops, G. Jonsson, Macromol. Chem. Phys. 1998, volume 199, page 2723; M. A. Harmer, Langmuir 1991, volume 7, page 2010. In these examples, the ester linkage chosen to connect PEG to the underlying membrane is convenient synthetically, but ultimately not ideally suited for water purification applications due to the hydrolytic instability of esters. Composite membranes prepared from polymer blends, such as polyether sulfone/poly(vinylpyrrolidone) (see, R. M., Boom, I. M. Wienk, T. van den Boomgaard, C. A. Smolders, J. Membr. Sci. 1992, volume 73, page 277), and poly(vinylidene fluoride) (PVDF)/PVDF-PEGylated PMMA (see, J. F. Hester, A. M. Mayes, J. Membr. Sci. 2002, volume 202, page 119; J. F. Hester, P. Benerjee, Y. Y. Won, A. Akthakul, M. H. Acar, A. M. Mayes, Macromolecules 2002, volume 35, page 7652), also provide examples of materials that exhibit reduced fouling relative to conventional commercial membranes. However, potential disadvantages of the blending approach include leaching of the anti-fouling additive over time, inferior physical and mechanical properties of the blended material compared to a homogeneous polymer, and the need for polymer blending steps in the membrane fabrication process.
There remains a need for water purification membranes that are hydrolytically stable and exhibit improved resistance to fouling.